To the Editor.—Our concept of the pathogenesis of psoriasis ascribes a major role to Malassezia ovalis (Pityrosporum ovale) residing in the hair roots of so-called seborrheic areas. In an attempt to reduce the numbers of those organisms, we have used oral ketoconazole therapy in patients with psoriasis and reported apparent benefit in a small group of patients.1Further use of oral ketoconazole therapy in patients with psoriasis has been followed up in some patients by clearing of psoriatic patches in nonseborrheic areas of the skin, in sites where few or no M ovalis reside. This suggested the possibility that ketoconazole was working systemically, perhaps on yeasts residing in the gut.Meanwhile, Baker2 reported improvement in the condition of patients with both psoriasis and inflammatory bowel disease when treated with oral nystatin. Oral nystatin is poorly absorbed from the gut and its effect is considered to be almost